The process of embossing is widely used in the production of consumer goods. Manufacturers use the embossing process to impart a texture or relief pattern into products made of textiles, paper, synthetic materials, plastic materials, metals, and wood. Embossing a product can enhance the visual perception or improve the performance of the product. For example, embossing a paper product can result in a visually pleasing pattern on the paper or in the increased bulk and absorbency of the product.
Embossing is the act of mechanically working a substrate to cause the substrate to conform under pressure to the depths and contours of a pattern engraved or otherwise formed on an embossing roll. Embossing is accomplished by passing a substrate, or web, through one or more patterned embossing rolls set to apply a certain pressure and penetration depth to the web. As the web passes the embossing rolls, the pattern on the rolls is imparted onto the web.
The patterns on the embossing rolls can be mated or non-mated. In a pair of mated embossing rolls, the pattern on one of the rolls compliments identically, or "mates," with the pattern on the other of the mated rolls. The pattern on a non-mated embossing roll does not match identically with the pattern on the other roll. Depending on the desired results, either type of embossing roll can be used.
A problem encountered during the embossing process is caused by the buildup of deposits on the embossing rolls. As the embossing roll works a web, fiber, stickies, and other deposits from the web stick to the surface of the embossing roll. The accumulation of deposits on the pattern on the embossing roll changes the depths and contours of the pattern that is imprinted into the web and impairs the embossing definition. Also, the unchecked accumulation of deposits might lead to vibrations in the embossing roll as it rotates with the web. This quick accumulation of deposits can damage or destroy a set of mated embossing rolls if the accumulated deposits are not removed from the rolls regularly. Thus, to ensure that the embossing process produces the desired results in a safe manner, the process must be stopped periodically to clean the deposits from the embossing rolls.
One approach to solving this problem is to attempt to clean loose fiber from the web before the web reaches the embossing roll. These types of web cleaning devices are widely described in the patent literature. A comprehensive, but non-exhaustive list includes U.S. Pat. Nos. 5,577,294; 5,490,300; 5,466,298; 5,304,254; 4,783,947; 4,643,775; and 4,594,748. However, these devices remove few, if any, stickies or pitch that is firmly embedded in the web. Furthermore, these inventions do not remove 100% of the loose fiber present.
Devices for cleaning deposits from embossing rolls, such as the device disclosed in U.S. Pat. No. 4,852,209, are very complicated and expensive. They usually require spraying water or solvent on the rolls and then removing the water or solvent. Usually there is some residual solvent which can itself lead to plugging of an emboss roll. Brush rolls are not an appropriate alternative because they do not remove firmly embedded deposits and the brush bristles fall off and end up in the product.
Another potential solution to the accumulation of deposits problem is to apply a lubricant solution to the surface of the embossing roll using spray nozzles. The lubricant prevents some fiber and sticky build up, by lubricating the surface of the embossing roll so that deposits do not attach to the surface.
However, using a spray nozzle to apply the lubricant or solvent solution creates additional problems. To cover the entire surface of the embossing rolls, the spray nozzles must usually be positioned 6 to 8 inches from the surface of the roll and 6 to 8 inches apart. With this positioning, the spray nozzles create a mist of lubricant around the embossing roll. Some of the mist stays in the air and the remainder settles on the embossing roll, the surrounding equipment, or on the floor, thereby wasting a significant amount of the sprayed material. The mist in the air creates a breathing hazard and the mist that settles on the floor creates other safety hazards. In addition to wasting the sprayed chemical and creating potential breathing and safety hazards, the mist leaves an uneven coating on the surface of the embossing roll. The uneven coating of lubricant is ineffective in preventing the accumulation of deposits because the surface areas having a thicker coating of lubricant actually trap the deposits. Also, the deposits stick to the areas of the embossing roll that do not get enough lubricant.
In light of the foregoing, there is a need for a device and a method by which the embossing rolls can be kept clean of deposits during the embossing process.